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I picked up a New Braunsfels offset on CL for $50, it's in good shape and was only used 3 times for steaks. Here's the beaut as I got it.

I took it home did a little cleaning and tested it for leaks using charcoal and chips, I found it has a few door leaks and it cooks hot especially next to the firebox. After researching mods I noticed the FB(fire box) was mounted higher than it need to be and the FB opening was too big. I decided to try to make it more like the Old Country brand offsets.

Here's a screenshot from Tom's Test Kitchen reviewing his Old Country Pit, you can see the size opening compared to this COS(cheap offset smoker).

First was to lower the FB and decrease the size of the FB exhaust into the CC(cooking chamber). I used plate steel and bolted it together in case this didn't work. I took it apart and started marking for the new FB outlet and pre drilled all the mounting bolts. I don't have anymore leaks between the FB and CC.
I got it mounted back up and you can see how much I closed off.

Next I cut a heat deflector and made baffle/tuning plates

I didn't like the FB door, it had leaks all around because they didn't install plate on the inside to get a good seal.

I used the top of an old barrel and closed off most of the door, the vent still works fine.

I ordered a new 4" stack to move it to the side at grate level but wanted to test it before doing that mod.
I cleaned it up, sprayed it down with canola oil, set a water pan on the tuning plate filled with bacon chunks and water, cooked some pork fatty pieces and added a Sprite can to lower the stack to grill level. Let the seasoning/testing begin.
I let it go for about 6 hours and checked it every once in a while to add another log and check the temps. Here's some temps when the fire dropped, I'm impressed it's holding a steady temp across all the grates.
Left side near the FB
Right side near the stack
Factory gauge on the door

The seasoning worked great, it looks like it's ready to cook now.

Later that day I fired it back up to smoke some stuffed turkey wings, it held temp perfect(about 250) using very little wood for this 5 hour cook.
A few hours later
It's blurry but it sitting at 250
I cooked 2 hours then wrapped in foil with apple juice for another 2 hours, sticky juicy snacks.

The real test will come his weekend when I do a butt or brisket. I'll make a decision then about adding the 4" stack but I think I'll add it just to get that space on the grate and it should pull harder for better heat.

Any thoughts/ideas on the 4" stack? I did use a few pit building calculators and the FB exhaust opening and stack size is what I came up with.